A hearing aid is provided which has at least one input transducer for providing an input signal, at least one signal processing channel receiving at least a portion of said input signal, a hearing aid processor for processing said portion of said input signal to produce at least one output signal, an output transducer responsive to said output signal, and a data logger receiving said portion of said input signal for logging of input signal data. The data logger comprises a characterization unit for characterising and logging parameters of the input signal data, and a memory unit for storing said parameters.